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PEM was a proposed IETF standard for secure email. It depended on a single root certificate for its public key infrastructure (PKI), which was impractical and had its own problematic implications for security.

PGP started as a "proof of concept" for a less centralized "web of trust" PKI, and proved to be much more practical, finding widespread adoption and eventually founding the OpenPGP standard, while PEM faded into obscurity.

So basically PEM and PGP were competing protocols for encrypting emails, and PGP "won" while PEM "lost".